Magnetic materials find widespread use in modern technology and are to be found in nearly all electro-mechanical apparatus. The performance of magnetic materials in respect of their secondary parameters, such as coercivity and energy product, has improved greatly over the last century. There has nevertheless been little improvement in the most fundamental property, i.e. saturation magnetisation, which determines the strength of produced magnetic field. The most magnetic material for use in electro-mechanical apparatus, i.e., Fe60Co40 alloy, has been available since the 1920s and until recently there has been no material found with a higher magnetisation.
The most direct measure of saturation magnetisation is the magnetic moment per atom which is specified in Bohr magnetons (μB). The magnetic moment for pure Fe is 2.22μB per atom whereas for Fe80Co40 alloy the magnetic moment is 2.45μB per atom. The latter value, i.e., 2.45μB per atom, is termed the Slater-Pauling limit and was believed to be the ultimate magnetisation available from transition metal alloys. Generally the efficiency of electro-mechanical apparatus improves as the square of the magnetisation of the magnetic material. Even small increases in magnetisation are therefore valuable especially in green technologies such as electric vehicles and wind turbines.
Upon development in the early 1990s of gas-phase nanoparticle sources capable of depositing nanoparticles with diameters in the range of 1 to 5 nm it was discovered that the magnetic moments per atom of Fe, Co and Ni nanoparticles with diameters no more than about 5 nm are significantly higher than for bulk structures formed from the same material. In view of this, magnetic structures in which nanoparticles of one of Fe and Co are embedded in a matrix of the other of Fe and Co have been developed with such magnetic structures having a magnetisation which exceeds the magnetisation of Fe60Co40 alloy to thereby break the Slater-Pauling limit for the first time. FIG. 1A illustrates the formation of one such magnetic structure. As shown in FIG. 1A a magnetic structure 10 is formed by co-deposition on a substrate 12 of Fe nanoparticles 14 from a cluster source 16 and of Co matrix material 18 from a Molecular Beam Epitaxy (MBE) source 20. Co-deposition of Fe nanoparticles and Co matrix material results in a structure in which Fe nanoparticles are distributed through and embedded in the Co matrix. According to an alternative approach a magnetic structure in which Co nanoparticles are distributed through and embedded in an Fe matrix is formed by co-deposition of Co nanoparticles from the cluster source and of Fe matrix material from the MBE source.
The magnetic moment per atom for each of a structure having Fe nanoparticles in a Co matrix and a structure having Co nanoparticles in an Fe matrix are shown in FIG. 1B as a function of the Fe volume fraction. FIG. 1B also shows the Slater-Pauling curve for Fe60Co40 alloy as a function of the Fe volume fraction. As can be seen from FIG. 1B the best results are obtained from Co nanoparticles embedded in an Fe matrix which yields values approaching 3μB per atom. At lower Fe volume fractions the magnetic moment per atom for Fe nanoparticles embedded in a Co matrix exceeds the corresponding value defined by the Slater-Pauling curve. The improvement is seen because the fundamental building blocks of the material already have an enhanced magnetisation and also because the matrix itself has a nanostructure which leads to enhanced moments. More specifically there is a higher proportion of atoms at a surface or interface in a nanostructure (approaching 50% in the presently described structure) with each such atom having enhanced spin and orbital moments. On the other hand and as can be seen from the left half of the graph of FIG. 1B the magnetisation falls below the Slater-Pauling curve at Fe volume fractions of more than about 20% which is the percolation threshold.
The present inventors have appreciated that the high level of aggregation of Fe nanoparticles at higher levels of Fe volume fractions produces a phase separated mixture of macroscopic grains and the magnetisation falls as a consequence to a weighted average of the magnetic moments of Co (1.7μB per atom) and Fe (2.22μB per atom). This is the reason for the fall in the magnetisation to below the Slater-Pauling curve as seen in FIG. 1B. The present inventors have further appreciated that an improvement in performance may be gained by providing for an increase in the nanoparticle volume fraction without marked aggregation.
The invention has been devised in the light of the above mentioned appreciation. It is therefore an object for the present invention to provide an improved process of forming a magnetic structure comprising magnetic particles and in particular magnetic nanoparticles embedded in a matrix. It is a further object for the present invention to provide improved apparatus for forming a magnetic structure comprising magnetic particles and in particular magnetic nanoparticles embedded in a matrix. It is a yet further object for the present invention to provide an improved magnetic structure comprising magnetic particles and in particular magnetic nanoparticles embedded in a matrix.